Ink compositions for use in ink jet printers generally comprise deionized water, a water soluble or water miscible organic solvent, and a colorant. Frequently, the colorant is a soluble dye. Unfortunately, inks comprising soluble dyes can exhibit many problems, such as poor water-fastness, poor light-fastness, clogging of the jetting channels as a result of solvent evaporation and changes in the dye's solubility, dye crystallization, ink bleeding and feathering on the printed page, poor thermal stability, chemical instability and ease of oxidation. Many of these problems can be minimized by replacing the soluble dyes used in ink formulations with insoluble pigments. In general, pigments have superior properties when compared to dyes, including good water-fastness, good light-fastness, thermal stability, oxidative stability and compatibility with both coated/treated and plain papers.
In pigmented ink compositions, the insoluble pigment is generally stabilized in a dispersion by a polymeric component. Generally speaking, most pigment inks stabilized by polymers in aqueous media are based on an electrostatic stabilizing mechanism in which a hydrophobic group in the dispersant acts as an anchor adsorbed on the pigment particle surface through acid-base relation, electron donor/acceptor relation, Van der Waals forces, or physical adsorption. In this type of system, a hydrophilic group in the dispersant is extended into the aqueous medium to keep the dispersant soluble and to set up an electrosteric layer to prevent aggregation of the particles. This results in a competition in the dispersing process between the pigment particle and the polymer, the polymer and the solvent, and the pigment particle and the solvent.
In order to form a stable polymeric dispersion, several factors need to be considered. First, the polymer must be firmly anchored to the pigment particle surface to withstand shear force and the competition of other ingredients. This requires a careful match of the polarity of the pigment particle surface and the hydrophobic group in the dispersant. Second, there is a need to adjust the identity, length and weight of the hydrophobic group in the dispersant to fully cover the pigment particle, otherwise, the adsorbed polymer will act as a flocculent. Third, an electrosteric layer with requisite thickness around the particle to repulse aggregation is needed.
It is common to add solvents, surfactants, defoamers, biocides and other additives to pigmented ink to optimize its print quality, dry time and maintenance characteristics. These additives may compete with the anchor group in the dispersant to adsorb on the pigment particle surface, and may also lower the solubility of the polymer in the media, especially at higher temperatures, thereby destabilizing the dispersion system. Destabilization of pigment dispersions in inks can result in flocculation of the pigment in the nozzle of the ink jet printer which can adversely impact the printing process. Most prior art pigment dispersions will clog the nozzle of ink jet printers when left standing in the atmosphere for an extended period of time (e.g., about 6 hours). The result of these interrelated and competing forces is that it has been extremely difficult to formulate pigmented ink jet inks which simultaneously exhibit excellent stability, water-fastness and excellent print density yet exhibit minimal print head nozzle clogging.
Attempts have been made to minimize print head nozzle clogging by formulating inks with hygroscopic materials (humectants) to keep inks moist in print cartridge nozzles. High quantities of humectants keep nozzles clog-free and minimize print head nozzle clogging. Unfortunately, high quantities of humectants adversely affects ink in terms of viscosity, dry time and smudging. Therefore, there is a need for ink comprising humectant combinations which decrease dry time and smudging while keeping print head nozzle substantially free of clogs.
Wickramanayke et al., U.S. Pat. No 5,302,197, disclose ink compositions comprising pigment dispersion, aqueous carrier medium and a cosolvent mixture comprising a polyol/alkylene oxide condensate and a cyclic amide derivative.
Nagai et al., U.S. Pat. Nos. 5,810,915, 5,879,439 and 5,882,390, disclose ink compositions which may comprise water-soluble organic solvents such as polyhydric alcohols, alkyl and aryl ethers derived from polyhydric alcohols, nitrogen-containing heterocyclic compounds, amides, amines and sulfur-containing solvents.
Yamashita et al., U.S. Pat. No. 5,883,157, disclose the use of water-soluble organic solvents to prevent evaporation of water in ink for ink jet recording. Suitable solvents include polyhydric alcohols, glycol ethers, solvents containing sulfur and solvents containing nitrogen.
Schwarz, Jr., U.S. Pat. No. 5,882,389, discloses ink compositions comprising water, colorant and an oxazolidonone compound.
Hill, Jr. et al., U.S. Pat. No.3,859,388, disclose aralkyl mercaptans as effective polymerization molecular weight modifiers for the preparation of nitryl polymers.
Beach et al., U.S. Pat. Nos. 5,714,538 and 5,719,204, disclose ink dispersants which are graft copolymers comprising a hydrophilic polymeric segment, a hydrophobic polymeric segment incorporating a hydrolytically-stable siloxyl substituent, and a stabilizing segment, such as a reactive surfactant macromer, a protective colloid macromer, or a non-siloxyl hydrophobic monomer. Beach et al. disclose the graft copolymers are synthesized using chain transfer agents such as C.sub.4 -C.sub.18 alkylthiols, and exemplify a polymeric dispersant synthesized using dodecanethiol.
Hirayama et al., U.S. Pat. No. 5,782,968, disclose a method of producing a black pigment comprising the step of contacting a polymer with a carbon material at a temperature of 50.degree. C. to 250.degree. C. Hirayama et al. disclose the polymer may be synthesized using a photopolymerization initiator, a hexaaryl-bisimidazole compound or a hydrogen donor, such as an aromatic mercaptan compound or an aromatic amine compound.
Ma et al., U.S. Pat. Nos. 5,221,334 and 5,085,698, disclose particles of pigment stabilized in a block copolymer comprising a hydrophobic segment and a hydrophilic segment. Ma et al. further disclose that the hydrophobic segment is a hydrophobic homopolymer or copolymer of an acrylic monomer, and that the hydrophilic segment is a hydrophilic polymer or salt thereof of an acrylic monomer or a copolymer of an acrylic monomer.
There is a need for ink compositions with decreased dry time and dry smudging. Such compositions generally require improved humectant mixtures and/or lower levels of humectant. There is also a need for dispersants which can stabilize pigments in ink compositions which comprise lower levels of humectants.